SegmentedBegin

Marks an IngestPayload as the first request of a segmented (multi-block) ingest session.
Presence (not a bool) tells the ingest handler to take the segmented-begin branch and return a
[SegmentedBeginResponse].

{
    "block_budget": 0,
    "source_hash": "string",
    "total_blocks_hint": 0
}